The present invention relates generally to gas turbines, and more particularly a method, apparatus, and system for improving the efficiency of a land-based gas turbine by controlling the blade tip clearance. In a presently preferred embodiment of the invention, the blade tip clearance is reduced by heating a blade ring during a transient, non-steady state, period of operation of the turbine to increase the blade tip clearance during that period.
A gas turbine engine includes a rotary compressor for compressing the air flow entering the engine, a combustor in which a mixture of fuel and compressed air is burned to generate a propulsive gas flow, and a turbine that is rotated by the propulsive gas flow and is connected by a shaft to drive the compressor. The efficiency of a gas turbine depends in part on the clearance between the rotor blade tips and the surrounding engine casing or shroud, such as the clearance between the engine's turbine blades and the engine's turbine casing and the clearance between the engine's compressor blades and the engine's compressor casing. If the clearance is too large, more of the engine air flow will leak through the gap between the rotor blade tips and the surrounding shroud, decreasing the engine's efficiency. If the clearance is too small, the rotor blade tips may strike the surrounding shroud during certain engine operating conditions. Further background information is provided by U.S. Pat. No. 5,228,828, Jul. 20, 1993, titled "Gas Turbine Engine Clearance Control Apparatus" (Damlis et al.); U.S. Pat. No. 5,295,787, Mar. 22, 1994, titled "Turbine Engines" (Leonard et al.); and U.S. Pat. No. 5,219,268, Jun. 15, 1993, titled "Gas Turbine Engine Case Thermal Control Flange" (Johnson).
Typically, the cold clearance between the rotor blade tip and adjacent flow path outer diameter is set to minimize tip clearance during steady state and to avoid tip rubs during transient periods. A transient period, such as during a fast start, typically dictates the cold setting, and consequently the steady state tip clearance is greater than the minimum clearance possible. This results in extra leakage past the blades and reduced efficiency,.
A goal of the present invention is to improve on this situation by controlling the flow path outer diameter to increase the blade tip clearance during transients, and to thereby provide a reduced cold clearance and, consequently, a reduced steady state clearance with its associated efficiency improvement.